Distribution of vessel size,vessel density and xylem conducting efficiency within a crown of silver birch (<Emphasis Type="Italic">Betula pendula</Emphasis>)

Spatial patterns in vessel diameter, vessel density and xylem conducting efficiency within a crown were examined in closed-canopy trees of silver birch (Betula pendula). The variation in anatomical and hydraulic characteristics of branches was considered from three perspectives: vertically within a crown (lower, middle and upper crown), radially along main branches (proximal, middle and distal part), and with respect to branch orders (first-, second- and third-order branches). Hydraulically weighted mean diameter of vessels (D _h) and theoretical specific conductivity of the xylem (k _t) exhibited no vertical trend within the tree crown, whereas leaf-specific conductivity of the xylem (LSC_t) decreased acropetally. Variation in LSC_t was governed by sapwood area to leaf area ratio (Huber value) rather than by changes in xylem anatomy. The acropetal increase in soil-to-leaf conductance (G _T) within the birch canopy is attributable to longer path length within the lower-crown branches and higher hydraulic resistance of the shade leaves. D _h, k _t and LSC_t decreased, while vessel density (VD) and relative area of vessel lumina (VA) increased distally along main branches. A strong negative relationship between vessel diameter and VD implies a trade-off between hydraulic efficiency and mechanical stability of xylem. D _h and VD combined explained 85.4% of the total variation of k _t in the regression model applied to the whole data set. Xylem in fast-growing branches (primary branches) had greater area of vessel lumina per unit cross-sectional area of sapwood, resulting in a positive relationship between branch radial growth rate and k _t. D _h, k _t and LSC_t decreased, whereas VD increased with increasing branch order. This pattern promotes the hydraulic dominance of primary branches over the secondary branches and their dominance over tertiary branches. In this way crown architecture contributes to preferential water flow along the main axes, potentially providing better water supply for the branch apical bud and foliage located in the outer, better-insolated part of the crown.     

Xylitol Production from D‐Xylose at Different Oxygen Transfer Coefficients in a Batch Bioreactor

Conversion of D‐xylose to xylitol by Candida boidinii NRRL Y‐17213 was studied under anaerobic and oxygen limited conditions by varying the oxygen transfer coefficient k_La. Shake flask experiments were used to provide the preliminary information required to perform experiments in a bioreactor. The yeast did not grow under fully anaerobic conditions, but anaerobic formations of xylitol, ethanol, ribitol, and glycerol were observed as well as D‐xylose assimilation of 11 %. In shake flasks, with an initial D‐xylose concentration of 50 g/L, an increase in k_La from 8 to 46 h^–1 resulted in a faster growth, higher rate of substrate uptake and lower yields of products. The highest xylitol productivity (0.052 g/L h) was attained at k_La = 8 h^–1. At k_La = 46 h^–1, 98.6 % of D‐xylose was consumed and mainly converted to biomass. Using 130 g/L D‐xylose, k_La was varied in the fermenter from 26 to 78 h^–1. The percentage of consumed D‐xylose increased from 31 % at k_La = 26 h^–1 to 93–94 % at all other aeration levels. Biomass yield increased with k_La, whereas ethanol, ribitol, and glycerol yields exhibited an opposite dependence on the oxygenation level. The most favorable oxygen transfer coefficient for xylitol formation, in the fermenter, was k_La = 47 h^–1 when its concentration (57.5 g/L) surpassed ethanol accumulation by 3.6‐fold, and the glycerol plus ribitol by 10‐fold. Concurrently, xylitol yield and productivity reached 0.45 g/g and 0.26 g/L h, respectively. The volumetric xylitol productivity was affected more by changes in the aeration than the corresponding yield.     

Does acclimation in cavitation resistance due to mechanical perturbation support the pit area or conduit reinforcement hypotheses in Phaseolus vulgaris?

Two Phaseolus vulgaris L. cultivars were exposed to reduced water and stem mechanical perturbation treatments (flexing) to determine if acclimation to these treatments induced hydraulic changes, altered cavitation resistance and changed stem mechanical properties. Additionally, this study sought to determine if changes in cavitation resistance would support the pit area or conduit reinforcement hypotheses. Flexing reduced biomass, leaf area, xylem vessel area and hydraulic conductivity. One cultivar had greater measures of stem strength and cavitation resistance. Flexing increased cavitation resistance (P₅₀) but did not increase Young's modulus, rigidity or flexural strength on dried stems. Stem rigidity and basal diameter were correlated with leaf mass. The ratio of conduit wall thickness to span [(t/b)_h²] increased under high water and flexing treatments while rigidity decreased for one cultivar exposed to both flexing and lower water suggesting an inability to compensate for two simultaneous stresses. Although P₅₀ was not correlated with measures of mechanical strength, P₅₀ was correlated with vessel diameter, consistent with the pit area hypothesis. This study confirmed that mechanical perturbation can impact xylem structural properties and result in altered plant water flow characteristics and cavitation resistance. Long‐term hydraulic acclimation in these herbaceous annuals was constrained by similar tradeoffs that constrain hydraulic properties across species.     

CO2-forced evolution of plant gas exchange capacity and water-use efficiency over the Phanerozoic

The capacity of plants to fix carbon is ultimately constrained by two core plant attributes: photosynthetic biochemistry and the conductance to CO₂ diffusion from the atmosphere to sites of carboxylation in chloroplasts, predominantly stomatal conductance. Analysis of fossilized plant remains shows that stomatal density (number per unit area, D) and size (length by width, S) have fluctuated widely over the Phanerozoic Eon, indicating changes in maximum stomatal conductance. Parallel changes are likely to have taken place in leaf photosynthetic biochemistry, of which maximal rubisco carboxylation rate, V_cmax is a central element. We used measurements of S and D from fossilized plant remains spanning the last 400 Myr (most of the Phanerozoic), together with leaf gas exchange data and modeled Phanerozoic trends in atmospheric CO₂ concentration, [CO₂]_a, to calibrate a [CO₂]_a‐driven model of the long‐term environmental influences on S, D and V_cmax. We show that over the Phanerozoic large changes in [CO₂]_a forced S, D and V_cmax to co‐vary so as to reduce the impact of the change in [CO₂]_a on leaf CO₂ assimilation for minimal energetic cost and reduced nitrogen requirements. Underlying this is a general negative correlation between S and D, and a positive correlation between water‐use efficiency and [CO₂]_a. Furthermore, the calculated steady rise in stomatal conductance over the Phanerozoic is consistent with independent evidence for the evolution of plant hydraulic capacity, implying coordinated and sustained increase in gas exchange capacity and hydraulic capacity parallel long‐term increases in land plant diversity.     

Pigment organization and energy transfer in the green photosynthetic bacterium Chloroflexus aurantiacus

The transfer of excitation energy and the pigment arrangement in isolated chlorosomes of the thermophilic green bacterium Chloroflexus aurantiacus were studied by means of absorption, fluorescence and linear dichroism spectroscopy, both at room temperature and at 4 K. The low temperature absorption spectrum shows bands of the main antenna pigments BChl c and carotenoid, in addition to which bands of BChl a are present at 798 and 613 nm. Fluorescence measurements showed that excitation energy from BChl c and carotenoid is transferred to BChl a, which presumably functions as an intermediate in energy transfer from the chlorosome to the cytoplasmic membrane. Measurements of fluorescence polarization and the use of two different orientation techniques for linear dichroism experiments enabled us to determine the orientation of several transition dipole moments with respect to each other and to the three principal axes of the chlorosome. The Q_y transition of BChl a is oriented almost perfectly perpendicular to the long axis of the chlorosome. The Q_y transition of BChl c and the -carotene transition dipole are almost parallel to each other. They make an angle of about 40° with the long axis and of about 70° with the short axis of the chlorosome; the angle between these transitions and the BChl a Q_y transition is close to the magic angle (55°).Abbreviations BChl bacteriochlorophyll - CD circular dichroism - LD linear dichroism Dedicated to Prof. L.N.M. Duysens on the occasion of his retirement.     

Electron diffraction of Valonia cellulose. A quantitative interpretation

The crystal structure of native cellulose (Valonia) has been analyzed by electron diffraction. Possible models for the structure were refined by rigid-body least squares methods, which incorporated parameters defining the preferred orientation of the fibrils around their long axes in the cell wall lamellae. The structure was found to consist of an array of chains having the same sense (i.e., parallel), with packing parameters similar to those recently determined by X-ray diffraction. The eight-chain unit cell could be approximated adequately by a two-chain monoclinic unit cell with dimensions a = 8.18 Å, b = 7.84 Å, c = 10.38 Å (fiber axis), and γ = 97.04°; the space group is P2₁.     

Properties of deactivation gating currents in Shaker channels

The charge versus voltage relation of voltage-sensor domains shifts in the voltage axis depending on the initial voltage. Here we show that in nonconducting W434F Shaker K⁺ channels, a large portion of this charge-voltage shift is apparent due to a dramatic slowing of the deactivation gating currents, Ig_D (with τ up to 80 ms), which develops with a time course of ∼1.8 s. This slowing in Ig_D adds up to the slowing due to pore opening and is absent in the presence of 4-aminopyridine, a compound that prevents the last gating step that leads to pore opening. A remaining 10–15 mV negative shift in the voltage dependence of both the kinetics and the charge movement persists independently of the depolarizing prepulse duration and remains in the presence of 4-aminopyridine, suggesting the existence of an intrinsic offset in the local electric field seen by activated channels. We propose a new (to our knowledge) kinetic model that accounts for these observations.     

XYLEM ANATOMY AND HYDRAULIC CONDUCTANCE OF PSILOTUM NUDUM

Psilotum nudum (L.) Beauv. (Psilotopsida) has a simple, vascularized sporophyte with a dichotomously branching aerial axis. The number and lumen diameters of tracheids in the actinostele decrease in each subsequent branch, leading to an approximate halving of the measured hydraulic conductance (K_h) from segment to segment. To understand how the anatomy of P. nudum affects K_h, a biophysical model based on the Hagen-Poiseuille relation was developed that incorporated lumen diameter, tracheid taper, pit cavities, and pit membranes. Using a technique previously developed for ferns, pit membrane resistance was determined by measuring water flow before and after dissolving the pit membranes with cellulase. Measured K_h was in good agreement with K_h calculated with the model after excluding thick-walled late metaxylem tracheids that dye studies showed were nonconducting. Model simulations showed that the approximately 40% overlap observed for tracheids of P. nudum was in the range leading to greatest conductance and that K_h decreased to half for 20% overlap. The model also showed that the pit membranes account for an increasing percentage of total resistance to water flow as the lumen diameter increases. Thus, the removal of such primary wall material and the evolutionary origin of vessels would have substantially increased K_h.     

Crystallization,preliminary X-ray diffraction study,and crystal packing of a complex between anti-Hen lysozyme antibody F9.13.7 and Guinea-fowl lysozyme

The complex formed between the Fab fragment of a murine monoclonal anti-hen egg lysozyme antibody F9.13.7 and the het-erologous antigen Guinea-fowl egg lysozyme has been crystallized by the hanging drop technique. The crystals, which diffract X-rays to 3 Å resolution, belong to the monoclinic space group P2₁, with a = 83.7 Å, b = 195.5 Å, c = 50.2 Å, β = 108.5° and have two molecules of the complex in the asymmetric unit The three-dimensional structure has been determined from a preliminary data set to 4 Å using molecular replacement techniques. The lysozyme–Fab complexes are arranged with their long molecular axes approximately parallel to the crystallo-graphic unique axis. Fab F9.13.7 binds an anti-genie determinant that partially overlaps the epitope recognized by antilysozyme antibody HyHEL10. © 1993 Wiley-Liss, Inc.     

The holoparasitic endophyte Bdallophyton americanum affects root water conductivity of the tree Bursera simaruba

It has been reported that parasitic vascular plants (hemiparasites and holoparasites) may affect host fitness, but the effects produced by root endophyte holoparasitic species on its host have not been documented. Here the effect of the holoparasitic endophyte Bdallophyton americanum (R. Br.) Harms on the root conductivity of Bursera simaruba (L.) Sarg. was studied. Parasitized and non-parasitized root segments were sampled in the rainy and dry seasons in a dry coastal forest in central Veracruz, Mexico. Root diameter, hydraulic (K _h) and specific conductivity (K _s = K _h/root transverse area), percent loss of conductivity and reproductive specific conductivity (K _h /inflorescence or infructescense dry weight) data were obtained. The diameter and number of conductive and non-conductive vessels were recorded in parasitized and non-parasitized root segments in the dry season. Root diameters were not different between root types and seasons, but root specific conductivity was different both between seasons and root types. Specific conductivity on parasitized roots was 61% (wet season) and 85% (dry season) lower than that recorded for non-parasitized roots in the wet season. Root hydraulic conductivity was positively related with the biomass of reproductive structures of B. americanum in the wet season. The parasite appears to alter the xylem morphogenesis of the host, reducing vessel number by 40%, but not plugging or otherwise harming the conductive vessels, and resulted in no change in vessel diameter. Contrary to what has been reported to occur in some plant stems infected with hemiparasitic mistletoes, B. americanum decreases but does not eliminate conductivity to the distal plant parts.     

Relationship between plant hydraulic and biochemical properties derived from a steady-state coupled water and carbon transport model

There is growing evidence that plant stomata have evolved physiological controls to satisfy the demand for CO₂ by photosynthesis while regulating water losses by leaves in a manner that does not cause cavitation in the soil–root–xylem hydraulic system. Whether the hydraulic and biochemical properties of plants evolve independently or whether they are linked at a time scale relevant to plant stand development remains uncertain. To address this question, a steady‐state analytical model was developed in which supply of CO₂ via the stomata and biochemical demand for CO₂ are constrained by the balance between loss of water vapour from the leaf to the atmosphere and supply of water from the soil to the leaf. The model predicts the intercellular CO₂ concentration (C_i) for which the maximum demand for CO₂ is in equilibrium with the maximum hydraulically permissible supply of water through the soil–root–xylem system. The model was then tested at two forest stands in which simultaneous hydraulic, ecophysiological, and long‐term carbon isotope discrimination measurements were available. The model formulation reproduces analytically recent findings on the sensitivity of bulk stomatal conductance (g_s) to vapour pressure deficit (D); namely, g_s = g_ref(1 ? m × lnD), where m is a sensitivity parameter and g_ref is a reference conductance defined at D = 1 kPa. An immediate outcome of the model is an explicit relationship between maximum carboxylation capacity (V_cmax) and soil–plant hydraulic properties. It is shown that this relationship is consistent with measurements reported for conifer and rain forest angiosperm species. The analytical model predicts a decline in V_cmax as the hydraulic capacity of the soil–root–xylem decreases with stand development or age.     

Estimating the time course of pore expansion during the spike phase of exocytotic release in mast cells of the beige mouse

Our objective is to determine the time course of exocytotic fusion pore opening (P) in mast cells of the beige mouse from the measured efflux of the spike phase of exocytotic release (J). We show that a pore whose meridian or radius grows linearly with time cannot reproduce the efflux. We also show that a pore that opens very quickly [relative to the diffusivity of 5-hydroxytryptamine (5-HT)] and completely (P = π) also does not mimic the experimental efflux, and estimate maximum pore angles of 70(±20)°. We show that a larger class of opening functions reproduces the rising phase and part of the decay phase and calculate pore expansion rate, pore radius and pore angle, none of which can be readily measured. In the initial stages of the spike phase (50–200 ms) when the gel matrix has not expanded significantly, this model suggests that the pore radius increases exponentially with a time constant of 82(±62) ms with pore expansion reaching its maximum velocity of 20(±7) nm ms⁻¹. We conclude that the release process is dynamic and suggest that the velocity of pore opening (V) and the diffusivity of 5-HT (D), in addition to the size of the vesicle (R, radius), vary with time. We discuss assumptions and improvements to the model and propose that this methodology is applicable for determining P from measured J in other endocrine cells and neurons when D within the secretory vesicle is much less than D within the pore neck.     

三种锦鸡儿属植物水力结构特征及其干旱适应策略

水分胁迫是干旱半干旱区限制植物生长的主要因素。以干旱半干旱区的3种锦鸡儿属植物为研究对象,从生态适应策略角度来分析3种锦鸡儿植物产生生态分离的原因。对三种锦鸡儿属植物茎干叶片的显微结构、生理功能(导水率、光合速率以及水分利用效率)进行测定,并统计了3种锦鸡儿植株的形态特征,如一、二级枝的直径、长度、末端叶面积。结果表明:三种锦鸡儿属植物都能形成较小的导管直径来适应旱生环境,但是在导水结构上又表现出一定的差异性。中间锦鸡儿的导管直径最小,次脉密度和最大净光合速率最大;柠条锦鸡儿的导管直径、叶片厚度和比叶重(LMA)最大。小叶锦鸡儿在导水率下降50%时的水势(P_(50))最大,水分胁迫时极易发生栓塞,但正是由于导管的栓塞降低了水分运输效率,使其在旱生环境中能够通过减少水分的供应来降低水分的丧失,从而保证自身生长的水分需求;而中间锦鸡儿则主要通过减小导管直径来适应旱生环境;柠条锦鸡儿的水分利用效率最高,抗栓塞能力最强,抗旱性最好,同时柠条锦鸡儿可以通过减少蒸腾面积来减少水分的丧失。植物的导管直径大小、叶片厚度、LMA、叶脉密度对植物导水速率、光合速率等生理功能都有一定的影响。     

The impact of vessel size on vulnerability curves: data and models for within‐species variability in saplings of aspen,Populus tremuloides Michx

The objective of this study was to quantify the relationship between vulnerability to cavitation and vessel diameter within a species. We measured vulnerability curves (VCs: percentage loss hydraulic conductivity versus tension) in aspen stems and measured vessel‐size distributions. Measurements were done on seed‐grown, 4‐month‐old aspen (Populus tremuloides Michx) grown in a greenhouse. VCs of stem segments were measured using a centrifuge technique and by a staining technique that allowed a VC to be constructed based on vessel diameter size‐classes (D). Vessel‐based VCs were also fitted to Weibull cumulative distribution functions (CDF), which provided best‐fit values of Weibull CDF constants (c and b) and P₅₀ = the tension causing 50% loss of hydraulic conductivity. We show that P₅₀ = 6.166D^?0.3134 (R² = 0.995) and that b and 1/c are both linear functions of D with R² > 0.95. The results are discussed in terms of models of VCs based on vessel D size‐classes and in terms of concepts such as the ‘pit area hypothesis’ and vessel pathway redundancy.     

A New Preparation Method of Coenzyme A with Microorganisms

The structure of aggregates formed by heating dilute BSA solution was analyzed with the fractal concept using light scattering methods. BSA was dissolved in HEPES buffer of pH 7.0 and acetate buffer of pH 5.1 to 0.1% and 0.001% solutions, respectively, and heated at 95°C, varying the heating time t_a. The fractal dimension D_f of the aggregate in the solution was evaluated from static light scattering experiments. The polydispersity exponent τ and the average hydrodynamic radius <R_h> of the aggregates were calculated from dynamic light scattering experiments using master curves obtained by Klein et al. The values of D_f and τ of heat-induced aggregates of BSA at pH 7.0 were about 2.1 and 1.5, respectively, the values of which agreed with those predicted by the reaction-limited cluster–cluster aggregation (RLCCA) model. On the other hand, D_f of heat-induced aggregates at pH 5.1 was about 1.8, which agreed with that predicted by the diffusion-limited cluster–cluster aggregation (DLCCA) model. The dependence of <R_h> for the sample of pH 7.0 on t_a was similar to that of the polystyrene colloids reported previously.     

不同原生境的6种棕榈科植物叶片水力性状的对比研究

为揭示生长在湿润生境和季节性干旱生境棕榈科植物的水分生理生态适应的差异,在中国科学院华南植物园中选取3种原分布于湿润生境(SOMH)植物:澳洲轴榈(Licualaramsayi)、槟榔(Arecacatechu)和所罗门皱籽棕(Ptychosperma salomonense),以及3种原分布于季节性干旱生境(SODH)植物:大丝葵(Washingtonia robusta)、澳洲蒲葵(Livistona australis)和大蒲葵(L. saribus),对叶片和叶柄的结构与功能性状进行测定。结果表明,与SODH物种相比,3种SOMH植物的水力加权导管直径(Dh)、理论导水率(Kth)、膨压丧失点(TLP)、脆弱性指数(VI)和比叶面积(SLA)较高,而其叶脉密度(VD)较低。TLP、VI均与Kth呈显著正相关,说明水力效率和水力安全之间存在权衡关系。可见,SOMH棕榈的叶片水力效率高于SODH棕榈,但叶片水力安全低于SODH,这揭示了生长于不同生境棕榈科植物在水分生理生态策略上的差异。     

Time-resolved fluorescence polarization anisotropy of short restriction fragments: the friction factor for rotation of DNA about its symmetry axis

The time-resolved fluorescence polarization anisotropy (FPA) of ethidium dye intercalated in 43 and 69 base pair (bp) restriction fragments is measured, and the friction factor per bp for rotation of DNA about its symmetry axis is determined. The same value of the hydrodynamic radius, a = 12.0 ± 0.6 Å, is obtained for both the 43- and 69-bp fragments, but only when (1) the twisting correlation functions appropriate for such short filaments are used: (2) the correct amplitude is employed for the uniform tumbling mode decays: and (3) the data analysis is restricted to times after the internal bending modes have died away leaving just reduced amplitudes of the exponentially decaying uniform tumbling modes. The present value of the hydrodynamic radius is significantly larger than that implied by the cross-sectional area perpendicular to the symmetry axis. This strongly suggests that a significant fraction of water in the major and minor groves is rotating more or less rigidly with the DNA on this time scale. The correct expression for the amplitude D_n(∞) of the uniform mode decay of the tumbling correlation function, including the average over all sites to which the dye could bind, is derived in the appendix. The present theory for D_n(∞) is compared with that of Barkley and Zimm [(1979) J. Chem. Phys. 70 , 2991–3007], and with recent Brownian simulations of discrete wormlike chains by Allison and co-workers [S. A. Allison and J. A. McCammon (1984) Biopolymers 23 , 363–375; S. A. Allison (1986) Macromolecules 19 , 118–124].     

Position on slope,disturbance, and tree species coexistence in a Seasonal Semideciduous Forest in SE Brazil

We investigated the influence of position on a slope (plot relative elevation) and vegetation disturbance (the tallest tree height per plot) on community composition and diversity in a SE Brazilian Seasonal Semideciduous Forest (46°55′ W, 22°50′ S). Trees with dbh ≥5 cm were sampled in one hundred 10 × 10 m plots randomly placed in a 6.5-ha stand. Through partial Mantel test, floristic dissimilarities among plots (Jaccard index computed with species abundance in each plot) were correlated with environmental distances among plots (Euclidian distance index computed with relative elevation and the tallest tree height values in each plot). Relative elevation and the tallest tree per plot height were individually correlated with floristic gradients expressed by PCA axes scores using Pearson’s correlation coefficient. Through resampling, we compared diversity (richness, Berger-Parker D and Shannon H′) among plots in the drier (up) and moister (low) ends of the slope. Floristic dissimilarities were significantly correlated with environmental distances even after geographic distances among plots have been partialled out (r _m = 0.1274, p < 0.001). The first two PCA axes accounted for 22% of the total variance. After Bonferroni and Dutilleul’s corrections, axis 1 showed a marginally significant correlation with plot relative elevation (r = − 0.4097, p = 0.0309), and axis 2 was significantly correlated with the tallest tree height per plot (r = 0.2953, p = 0.0106). Position on the slope and vegetation disturbance were reliable predictors of community composition, thus suggesting the operation of niche assembly organizing processes. Richness and diversity (H′) decreased and dominance (D) increased with elevation on the slope. Dominance increase from D ₍₃₀₀₎ = 0.11 (confidence interval = 0.091–0.131) to D ₍₃₀₀₎ = 0.19 (CI = 0.165–0.210) surpassed the expected dominance increase based on the reduction of richness alone: D ₍₃₀₀₎ = 0.13 (CI = 0.110–0.140), thus highlighting the niche partitioning assembly of the community, especially among abundant species. Given the great amount of floristic variability remaining unexplained, stochastic processes, such as those related to dispersal limitation, may also have influence on the community composition. Therefore, both niche assembly and chance events can operate even on a fine local scale.     

Stomatal responses to changes in vapor pressure deficit reflect tissue‐specific differences in hydraulic conductance

The vapor pressure deficit (D) of the atmosphere can negatively affect plant growth as plants reduce stomatal conductance to water vapor (g_wv) in response to increasing D, limiting the ability of plants to assimilate carbon. The sensitivity of g_wv to changes in D varies among species and has been correlated with the hydraulic conductance of leaves (K_leaf), but the hydraulic conductance of other tissues has also been implicated in plant responses to changing D. Among the 19 grass species, we found that K_leaf was correlated with the hydraulic conductance of large longitudinal veins (K_lv, r² = 0.81), but was not related to K_root (r² = 0.01). Stomatal sensitivity to D was correlated with K_leaf relative to total leaf area (r² = 0.50), and did not differ between C₃ and C₄ species. Transpiration (E) increased in response to D, but 8 of the 19 plants showed a decline in E at high D, indicative of an ‘apparent feedforward’ response. For these individuals, E began to decline at lower values of D in plants with low K_root (r² = 0.72). These results show the significance of both leaf and root hydraulic conductance as drivers of plant responses to evaporative demand.     

A critical appraisal of a combined stomatal-photosynthesis model for C3 plants

Gas-exchange measurements on Eucalyptus grandis leaves and data extracted from the literature were used to test a semi-empirical model of stomatal conductance for CO₂ g_Sc=g_o+a₁A/(c_s-I) (1+D_s/D_o)] where A is the assimilation rate; D_s and c_s are the humidity deficit and the CO₂ concentration at the leaf surface, respectively; g₀ is the conductance as A → 0 when leaf irradiance → 0; and D₀ and a₁ are empirical coefficients. This model is a modified version of g_sc=a₁A h_s/c_s first proposed by Ball, Woodrow & Berry (1987, in Progress in Photosynthesis Research, Martinus Mijhoff, Publ., pp. 221–224), in which h_s is relative humidity. Inclusion of the CO₂ compensation point, τ, improved the behaviour of the model at low values of c_s, while a hyperbolic function of D_s for humidity response correctly accounted for the observed hyperbolic and linear variation of g_sc and c_i/c_s as a function of D_s, where C_i is the intercellular CO₂ concentration. In contrast, use of relative humidity as the humidity variable led to predictions of a linear decrease in g_sc and a hyperbolic variation in c_i/c_s as a function of D_s, contrary to data from E. grandis leaves. The revised model also successfully described the response of stomata to variations in A, D_s and c_s for published responses of the leaves of several other species. Coupling of the revised stomatal model with a biochemical model for photosynthesis of C₃ plants synthesizes many of the observed responses of leaves to light, humidity deficit, leaf temperature and CO₂ concentration. Best results are obtained for well-watered plants.